What is :out after a shell call to execute some command?

Considering:

user=> (require '[babashka.process :refer [shell]])
nil
user=> (def j
  (-> (shell {:out :string :err "/dev/null"} "ls")
      :out some-func))

I understand that {:out :string :err "/dev/null"} is a map of a kind of options used to mold behavior for the shell command to run.

Then, at the next line.

:out, I understand it's a keyword that corresponds to stdout. I also know that it can be used to retrieve the corresponding value from a map.

But I don't understand how :out some-func can be used to call the function some-func.

Where does the macro -> place the result of the line (shell ...)? I suppose that after :out. So, the :out value would be extracted from the map of results of the (shell...) line, but how is it used as argument to some-func?

It is not even inside parenthesis. Why not?

The so-called "thread first" -> macro does two things:

1. It makes sure that every form passed to it is wrapped in (...)
2. It chains the forms so that (-> (a) (b) (c) ...) becomes (... (c (b (a))))

So in your case, the result is (some-func (:out (shell ...))).